This invention relates to a dielectric insulator separated substrate for semiconductor integrated circuits, which includes a large number of monocrystalline semiconductor island regions in which circuit elements are to be formed and a polycrystalline semiconductor support region for supporting fixedly the island regions.
In a semiconductor integrated circuit, circuit elements such as resistors, diodes, transistors, thyristors and the like may be formed integrally in island regions while being required to be electrically separated from each other. For this, a large number of island regions are electrically insulated from each other and from a support region which supports them. One of the insulating methods uses dielectric materials and a substrate prepared according to this method is called a dielectric insulator separated substrate (hereinafter referred simply to as DI substrate).
The DI substrate, however, undergoes curveness deformations during its preparation process and upon the production of semiconductor integrated circuits. The deformation brings about various defects including cracks in the substrate, degraded accuracy of metal deposition for the electrodes, degradation of the withstand voltage and fluctuations in characteristics of the circuit elements.
The U.S. Pat. Nos. 4,017,341; 4,079,506; 3,967,309; and 4,173,674 assigned to the same assignee of the present application describe in detail causes for the curveness deformations of the DI substrate and propose a countermeasure therefor. There are two types of curveness deformations of the DI substrate. In one type, the DI substrate is deformed convexly toward the side of the monocrystalline semiconductor island regions. In the other type, the DI substrate is deformed convexly toward the side of the polycrystalline semiconductor support region. The curveness deformation of the former type is caused by the difference in thermal expansion coefficient between the monocrystalline island regions and the polycrystalline support region. In the latter type, the deformation results from the wedge action due to oxygen diffused into the polycrystalline support region during a diffusion step of p- or n-type impurity into desired parts of mono-crystalline island regions.
The prior patents set forth above proposed to provide a film for preventing the diffusion of oxygen into the surface of the polycrystalline layer of the support region and/or a film for compensating for the difference in thermal expansion coefficient between the island regions and the support region.
U.S. Pat. No. 4,173,674 proposes the provision of a thin polycrystalline outer layer. The thin polycrystalline outer layer is formed by reducing a thick polycrystalline layer. The thickness of the outer layer before reducing its thickness is generally at least 30 .mu.m. If this thickness is too small, part of an inner thick polycrystalline layer may be exposed by reducing the thickness, which is caused by the curveness of the substrate. When the thick inner polycrystalline layer is exposed, the substrate will be deformed by the action of oxygen during the doping step of an impurity.
In general the ultimate thickness of the substrates is about 400 to 500 .mu.m. The substrates produced by the process disclosed in, such as, U.S. Pat. No. 4,173,674 have a curveness of 60 to 100 .mu.m, when a diameter of the substrate is 50 mm. After the polishing of the monocrystalline layer to form the island regions, the opposite face of the substrate is polished to obtain a flat surface. In order to carry out the second polishing, i.e. the reduction in thickness of the polycrystalline layer, the substrate, which has been subjected to the first polishing, i.e. the formation of island regions is supported by suction; however, there remains a curveness of about 20 .mu.m.
Since the partial growth of the polycrystalline layer tends to occur, the over-all difference in thickness of the substrate becomes about 30 .mu.m or more. The difference in local thickness of the substrate will increase as the diameter of the substrate increases. Accordingly, if the outer polycrystalline layer is 30 .mu.m thick or more, which is considered to be a sufficient thickness to cover the local difference of the substrate, there is a possibility that a part of the thick polycrystalline outer layer may remain unpolished. Therefore, the remaining thick polycrystalline outer layer can be a cause of deformation of the substrate due to oxygen diffusion during the doping step of an impurity.
It is an object of the present invention to provide a process of manufacturing a dielectric separated substrate by which a possibility of remaining a thick polycrystalline outer layer is eliminated.
The present invention consists in that a lamination of thin polycrystalline layers and dielectric films interposed therebetween is formed on a thick inner polycrystalline layer through a dielectric film. According to a process for producing a polycrystalline layer on a dielectric layer in gas phase, such as disclosed in U.S. Pat. No. 4,017,341, thin polycrystalline layers each of which is 10 .mu.m thick can be produced. Therefore, if more than three layers of such lamination are provided, although the thickness of about 30 .mu.m is reduced in order to obtain the substrate with a flat surface, a part of the lamination of the thin polycrystalline layers still remains and the polycrystalline layer of less than 10 .mu.m is exposed. Accordingly, the exposure of the thick inner polycrystalline layer, which adjoins the remaining thin outer polycrystalline layers through a dielectric film, can be avoided. The entire thickness of the lamination is necessary to cover the thickness for machining which is determined by a local difference in the thickness and the machining tolerance.
Other objects and features of the present invention will be apparent from the following description, taken in conjunction with the accompanying drawings in which: